EPSILON AND (Epsilon Andromedae). To the eye alone, stars give few
clues clue as to their natures. The only observables to the naked
eye are apparent brightness (which depends on real luminosity and
distance), and subtle color (reddish ones quite readily popping
out). Actual motions relative to each other are insensible. Most
of the stars of the nightly sky are in more or less circular orbits
about the center of our Galaxy, moving relative to each other by
only 10 to 20 kilometers per second. Not so Epsilon Andromedae,
which shines in western Andromeda
not far from the Great Square of
Pegasus. While looking much like the stars around it, it is a
local visitor from a distant neighborhood. This fourth magnitude
(4.37) class G (G8) giant star, with a temperature of 4930 Kelvin,
lies 169 light years away, from which we find a luminosity 52 times
that of the Sun, a radius 9.9 times solar,
and a rotation period less than twice the solar 25 day period.
Direct measure of its angular diameter gives a radius of 9.8 solar,
showing that all the parameters are right on the mark. From the
luminosity and temperature, Epsilon And is a 2.4 solar mass "clump
star," one that has settled in as a mid luminosity giant as it
quietly fuses the helium in its core into a mixture of carbon and
oxygen. Only 650 million years old, it began life as a class B9
hydrogen-fusing dwarf. Epsilon's claim to non-ordinary stature
lies in its motion. Moving away from us at 84 kilometers per
second, and across the line of sight at 83, its total velocity
relative to the Sun is a very high 118 kilometers per second. The
Sun orbits within our Galaxy at relatively constant distance of
28,000 light years from the center (with an orbital speed of about
220 kilometers per second). On a highly elliptical orbit, Epsilon
And comes to us from way inside the Galaxy, at a minimum distance
from the center of 14,000 light years, and is passing us, heading
for the distant outer Galactic neighborhood 35,000 light years from
the center. Typical of high-speed stars, it is also low in metals,
its iron content only a quarter that of the Sun (the Sun actually
being somewhat high for its own neighborhood).